The enzyme thyroxine 5'-deiodinase (T4D) activates thyroxine (T4) by conversion to triiodothyronine (T3) in target tissues. The separate enzyme iodothyronine 5-deiodinase (ITD) inactivates T4 and T3 by conversion to reverse-T3(rT3), and 3,3'-diiodothyronine respectively. T4D is present in rat brain and pituitary, and ITD is present in rat brain, but not pituitary. Subcellular localization of these particulate brain enzymes will be determined using discontinuous sucrose gradient fractionation, combined with assays for plasma membrane, mitochondrial amd microsomal marker enzymes. Purification of T4D from hypothyroid rat pituitary and brain will be attempted beginning with detergent solubilization followed by affinity chromatography. Purification of brain ITD will be attempted in the same way, using neonatal brain tissue, which has very high activity. In physiological studies, the effect of food deprivation on brain T4D and ITD activity will be ascertained in adult and neonatal rats, as will the effects of congenital hypothyroidism in offspring of pregnant rats fed goitrogens. The overall goal is to understand more fully how these enzymes modulate the expression of thyroid hormone action.